Water absorbent resins (SAPs/Super Absorbent Polymers) are water-swellable, water-insoluble polymer gelling agents, and are frequently used mainly for disposable uses as hygiene products such as disposable diapers and sanitary napkins, water retention agents for agriculture and horticulture, or industrial water stopping materials, or the like. Furthermore, various monomers and hydrophilic polymers have been suggested as raw materials for water absorbent resins, and polyacrylic acid (salt)-based water absorbent resins using acrylic acid and/or a salt thereof as a monomer are most frequently used in industries for their high water absorbent abilities.
Such water absorbent resins are produced through a polymerization step, a drying step, (where necessary, a step for removing an undried product), a pulverizing step, a classification step, a surface cross-linking step and the like (Patent Literatures 1 to 5). In accordance with the improvement of performances of disposable diapers as main intended use, many functions are required also for water absorbent resins. Specifically, not only a mere high water absorption capacity, but also many physical properties such as gel strength, water soluble component, water absorbent speed, water absorption capacity under load, liquid permeability, particle size distribution, urine resistance, antibacterial property, impact resistance, powder body fluidity, odor eliminating property, anticoloring property, and low powder dust are required for water absorbent resins. Therefore, many technologies for improvement such as a surface-cross-linking technology, additives, and modification of a production process are suggested by Patent Literatures 1 to 32 and others.
Among the above-mentioned physical properties, liquid permeability has been especially considered as a more important factor in accordance with the increase of the use amounts of water absorbent resins in disposable diapers (for example, 50% by weight or more) in recent years, and many methods for improving liquid permeability under loading and liquid permeability without loading such as SFC (Saline Flow Conductivity/Patent Literature 6) and GBP (Gel Bed Permeability/Patent Literatures 7 to 9) and improved technologies have been suggested.
Furthermore, many suggestions in which a plurality of parameters including liquid permeability are combined have been made, and for example, a technology for defining an impact resistance (FI) (Patent Literature 10), a technology for defining a water absorbent speed (FSR/Vortex) and the like (Patent Literature 11), a technology for defining a product of a liquid diffusion performance (SFC) and a core absorption amount after 60 minutes (DA60) (Patent Literature 12) are known.
Furthermore, as methods for improving liquid permeability such as SFC or GBP, a technology of adding gypsum before polymerization or during polymerization (Patent Literature 13), a technology of adding a spacer (Patent Literature 14), a technology of using 5 to 17 mol/kg of nitrogen-containing polymer having nitrogen atoms that can be protonized (Patent Literature 15), a technology of using a polyamine and a polyvalent metal ion or a polyvalent anion (Patent Literature 16), a technology of coating a water absorbent resin having a pH of lower than 6 with a polyamine (Patent Literature 17), and a technology of using a polyammonium carbonate (Patent Literature 18) are known. Furthermore, a technology of using a polyamine with a soluble component of 3% by weight or more, and technologies of defining a wicking index (WI) or a gel strength (Patent Literatures 19 to 21) are known. In addition, a technology of using a polyvalent metal salt while controlling methoxyphenol, which is a polymerization inhibitor during polymerization, so as to improve coloring and liquid permeability (Patent Literatures 22 and 23) is also known.
In addition, as methods for improving liquid permeability in a pulverizing step and a classification step, a technology of controlling a bulk specific gravity to be high by polishing particles (Patent Literature 24), a technology of assembling two or more classified polymers to give one classified polymer (Patent Literature 25), and a technology of removal of electricity (Patent Literature 26) are also known.
Especially, as methods for sieve classification of a water absorbent resin, a technology of heating or insulation (Patent Literature 27), a technology of reducing a pressure (Patent Literature 28), a technology of using a guide apparatus (Patent Literature 29), a technology of increasing an opening of a sieve for separating coarse particles after post-cross-linking (secondary cross-linking) (Patent Literature 30), a technology of combining a plurality of different classification steps (Patent Literature 31), and a technology of using a ball cleaning apparatus (Patent Literature 32) are known.